A widely used voltage reference supply is a band-gap circuit, which has typically been used to provide a low reference voltage with stability in the presence of temperature variations and noise or transients. In one form of band-gap circuit, known as a Brokaw circuit and described in the article “A simple Three-Terminal IC Bandgap Reference” in IEEE Journal of Solid-State Circuits, vol. SC9, no 6, December 1974, two groups of junction-isolated bipolar transistors run at different emitter current densities. The difference in emitter current densities produces a related difference between the base-emitter voltages of the two groups. This voltage difference is added to the base-emitter voltage of the transistor with higher emitter current density with a suitable ratio defined by a voltage divider. The temperature coefficient of the base-emitter voltage is negative and tends to compensate the positive temperature coefficient of the voltage difference.
A Brokaw band-gap circuit exhibits good stability and accuracy compared with other known circuits but still suffers from residual process dispersion, variability and temperature drift caused, for example, by mismatch of the mirror currents and base currents, especially when PNP transistors are used, which have low beta (collector-to-base current gain). PNP vertical transistors are preferred however for low power applications, to reduce parasitic effects in NPN vertical transistor integrated circuits, where parasitic horizontal transistor structures are formed by the different buried PN junctions, and high frequency current injection occurs due to DPI (direct power injection), with high frequency currents induced in the transistor collectors by parasitic capacitances at the buried PN junctions.
Especially, a standard Brokaw band-gap circuit also suffers from some inaccuracies due to dispersion of parameters due to manufacturing tolerances. While some of these sources of errors can be corrected during manufacturing, for example by trimming the products, such corrective actions do not give optimal results and increase manufacturing cost. Various circuits have been proposed with a view to reducing the sources of reference voltage inaccuracy in reference voltage circuits and also to ensuring low quiescent current.
The article “A curvature-corrected low-voltage bandgap reference” by Gunawan, M.; Meijer, G. C. M.; Fonderie, J.; Huijsing, J. H.; in the IEEE Journal of Solid-State Circuits Volume 28, Issue 6, June 1993 Page(s):667-670 and US patent specifications 20050122091, U.S. Pat. No. 5,081,410, 20050035813 and U.S. Pat. No. 6,172,555 describes various derivatives of the Brokaw circuit.
Our copending patent application PCT/IB2007/054337 describes a complementary bandgap circuit including two branches including respective groups of transistors of different emitter current conduction areas, each group including both pnp and npn transistors connected with their emitter-collector paths in series in the respective one of the branches. This arrangement provides an output voltage which is regulated to be substantially independent of variations in battery voltage and also to be independent of variations in operating temperature to a first order. The production dispersion of characteristics due to base current dispersion in the standard Brokaw circuit, notably due to production dispersion of the current gain of the transistors, can be reduced in this arrangement since the band-gap voltage Vbg is a function of the cumulated base-emitter voltage across two transistors of opposite type, a pnp and an npn with their base-emitter junctions connected in series and their emitter-collector paths in series. The cumulated voltage Vbep+n across each pair of transistors is the average of the base-emitter voltages of the two transistors of the pair, which statistically reduces the dispersion of the cumulated voltages. This applies to the dispersion of the value of Vbg and also to the dispersion of its rate of variation with temperature.
The article “A robust Smart Power Bandgap reference circuit for use in an automotive environment” in the IEEE Journal describes a bandgap circuit using both npn and pnp transistors but the circuit is not a complementary bandgap circuit, the pnp transistors being part of a differential amplifier.